I have made a macro which will work for Maxthon3 in KM 1.6.0 beta2. I just took the ie.kmm macro and changed all the names, values, etc from ie to maxthon. You can find the macro compressed with 7-Zip here.

Last part of post was to point you to the thread. Here is the macro. It is not installed, you just have to copy it into the macros folder and restart the browser.

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# K-Meleon Macros (http://kmeleon.sourceforge.net/wiki/index.php?id=MacroLanguage)
#
# browser.kmm v.0.1 --------------------------------------------------------------------
# Copy browser.kmm into your macros folder, usually c:\program files\K-meleon\macros
# Dependencies : main.kmm
# Preferences : kmeleon.plugins.macros.browser.
# Author : adodupan
#
#The end user adds an item on the menu for every browser or delete any item from the menu.
# It is not necessary to restart km, after selecting the exe file, the item will appear in the menu.
# To add items:
# Tools > Open in browser > Add a new browser
# To open a website you need to right click.
# ----------------------------------------------------------------------------------------
$macroModules=$macroModules."browser;";
$OnInit=$OnInit."_browser_01;_browser_02;";
$__browser_10="kmeleon.plugins.macros.browser.";

_browser_01{
setmenu("&Tools",popup,"&Open in browser",2);
setmenu("&Open in browser",macro,"&Add a new browser",_browser_04,0);
setmenu("&Open in browser",separator,-1);
}
_browser_02{
$List=getpref(STRING,$__browser_10."items"); $ExecMacros="_browser_03"; &ExecMacros_List;
}
_browser_03{
setmenu(DocumentOpenExternal,macro,"Open in ".$ListItem,"_browser_07(".$ListItem.")");
setmenu(LinkOpenExternal,macro,"Open in ".$ListItem,"_browser_08(".$ListItem.")");
setmenu(_Go_Open,macro,"Open in ".$ListItem,"_browser_09(".$ListItem.")");
setmenu("&Open in browser",macro,"Delete ".$ListItem,"_browser_05(".$ListItem.")");
}
_browser_04{
macroinfo=_("Set the name for the new browser!");
$__browser_11=prompt(_("The name will appear as an item in the menu"),_("Name of browser"),"The name should be unique!");
if($__browser_11=="The name should be unique!" or index($__browser_11,";")>-1 or index(getpref(STRING,$__browser_10."items"),$__browser_11.";")>-1) {
$__browser_12=confirm("Try again with another name!",_("Name of browser"),YESNO,QUESTION);
$__browser_12=="YES"?&_browser_04:0;
}else{
setpref(STRING,$__browser_10."items",getpref(STRING,$__browser_10."items").$__browser_11.";");
$__browser_13=$__browser_11; &_browser_06; &_browser_02;
}
}
_browser_05{
macroinfo=_("Delete from the list");
$__browser_14=$ARG;
setpref(STRING,$__browser_10."items",sub($__browser_14.";","",getpref(STRING,$__browser_10."items")));
getpref(STRING,$__browser_10."items")==""?delpref($__browser_10."items"):0;
delpref($__browser_10.$__browser_14.".path");
setmenu(DocumentOpenExternal,macro,"Open in ".$__browser_14,);
setmenu(LinkOpenExternal,macro,"Open in ".$__browser_14,);
setmenu(_Go_Open,macro,"Open in ".$__browser_14,);
setmenu("&Open in browser",macro,"Delete ".$__browser_14,);
}
_browser_06{
setpref(STRING,$__browser_10.$__browser_13.".path",promptforfile("",$__browser_13,"*.exe"));
if(getpref(STRING,$__browser_10.$__browser_13.".path")=="") {
delpref($__browser_10.$__browser_13.".path");
setpref(STRING,$__browser_10."items",sub($__browser_13.";","",getpref(STRING,$__browser_10."items")));
getpref(STRING,$__browser_10."items")==""?delpref($__browser_10."items"):0;
}
}
_browser_07{
macroinfo=_("Open in this browser");
$__browser_15=$ARG;
exec(getpref(STRING,$__browser_10.$__browser_15.".path")." ".$URL);
}
_browser_08{
macroinfo=_("Open in this browser");
$__browser_16=$ARG;
exec(getpref(STRING,$__browser_10.$__browser_16.".path")." ".$LinkURL);
}
_browser_09{
macroinfo=_("Open in this browser");
$__browser_17=$ARG;
$__browser_18=$URLBAR; $__browser_18==""?$__browser_18=$URL:0;
exec(getpref(STRING,$__browser_10.$__browser_17.".path")." ".$__browser_18);
}